1. Field of the Invention
The present invention relates to a thermal image recording apparatus which comprises a thermal print head provided with a heat sink.
2. Description of the Prior Art
In the thermal printing process, a dye-bearing donor ribbon is brought into contact with a dye-receiving print sheet at a print zone. Thermal printing is effected by contacting the donor ribbon with a multi-element print head which spans the ribbon in a direction transverse to the direction of ribbon travel. The print head typically comprises a linear array of closely spaced resistive heating elements, each being independently addressable by an applied voltage to heat that portion of the donor ribbon directly opposite and thereby cause dye to transfer from the donor ribbon to the print sheet. To maintain intimate contact between the donor ribbon and print sheet during this printing operation, the donor ribbon and print sheet are partially wrapped over the surface of a rotatably driven print drum.
The amount of picture element formation on the print sheet depends on the temperature of the heating elements, and on the temperature of the print head itself. The electric energy applied to the heating elements is controlled within a prescribed range by a control circuit. The temperature of the print head itself, on the other hand, is kept under control by dissipating the heat accumulated during printing by means of a heat sink. To that end, a stream of cooling air produced by a blower is directed over the fins of the heat sink.
The lifetime of a print head of the described type is limited since the rubbing contact of the backside of the dye donor ribbon with the resistive heating elements during each printing cycle causes accelerated wear of the heating elements. Typically, a print head is suited for performing 15,000 to 60,000 printing operations on an A4 cm format. For colour printing where usually patches of cyan, yellow and magenta dyes are printed in repeating series, this means that after approximately 5,000 to 20,000 prints the print head needs to be replaced.
Known print heads are mounted in the apparatus by means of a plurality of screws requiring quite some skill from the operator in order to perform the operations of holding the print head and mounting the screws simultaneously.
Furthermore, said screws must be tightened evenly in order to avoid torsion or bending of the printing head which can cause uneven pressure of the head on the dye-bearing donor ribbon in the print zone.